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Building Design and Construction Handbook - Merritt - Ventech!

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HEATING, VENTILATION, AND AIR CONDITIONING 13.83<br />

3. Interior VAV cooling with constant-volume, perimeter-heating, fan-coil units<br />

4. Interior VAV cooling with fan-powered perimeter heating or cooling<br />

5. Interior VAV cooling with perimeter VAV reheat<br />

6. Interior VAV cooling, double-duct, with VAV perimeter cooling <strong>and</strong> heating<br />

These systems vary widely in advantages <strong>and</strong> disadvantages <strong>and</strong> should be analyzed<br />

very carefully in the preliminary stages of design for each project.<br />

13.31.1 Zoning of <strong>Building</strong> Interiors<br />

Zoning is very important in the design stage <strong>and</strong> will contribute greatly to the<br />

success or failure of the HVAC system. An insufficient number of zones will lead<br />

to loss of temperature control in areas that do not have the controlling thermostat.<br />

The more zones utilized, the better will be the comfort conditions attained, but with<br />

some increase in initial costs.<br />

13.31.2 Air Distribution<br />

All types of diffusers (registers) may be used with VAV control units. St<strong>and</strong>ard<br />

diffusers with reduced air-flow rates, however, often cause ‘‘dumping’’ of discharge<br />

air directly downward with wide variations in space temperature. Many authorities<br />

recommend linear-slot-type diffusers to negate dumping. Such diffusers discharge<br />

air into the room with a horizontal flow that hugs the ceiling. This is called the<br />

co<strong>and</strong>a characteristic effect <strong>and</strong> is very effective in distributing supply air throughout<br />

a room.<br />

13.31.3 VAV Terminal Control Boxes<br />

For producing variable volume other than by modulation of the supply fan, terminal<br />

control units of many types may be used:<br />

Shut-off control diffusers are used for interior cooling. They are available with<br />

many features, such as shut-off operation, multiple slots, integral slot diffuser,<br />

electric-pneumatic or system-powered controls. They are available in 200- to<br />

800-cfm capacities, with choice of aspirating, unit-mounted pneumatic, system<br />

powered or electric thermostats. Savings are attributable to modulations of supplyair<br />

fans, which also provide complete control flexibility with reduced control costs.<br />

These diffusers are easy to install <strong>and</strong> are compatible with most ceiling systems.<br />

Fan-powered control units are generally used for perimeter <strong>and</strong> special-use<br />

areas. They feature VAV operation in the cooling mode. They are available with<br />

pressure-compensating controls with factory-installed hot-water or multistage electric<br />

coils that have pneumatic or electric controls. A built-in, side-mounted fan<br />

recirculates plenum air. Fan-powered units permit maximum modulation of the central<br />

fan as the control unit modulates to full shut-off VAV position. A terminal fan<br />

in the control unit is operated to maintain minimum flow in perimeter zones only.<br />

Heat generated by lighting fixtures is also used to assist in heating of the perimeter<br />

zones. Additional or supplemental heat is provided by unit-mounted hot-water or<br />

electric coils. No reheating of conditioned air is utilized.

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